1. Field of the Invention
This invention relates generally to the bulk measurement of trace contaminants in the surface layers of semiconductor wafers and dies, as well as material composition as a function of depth. More particularly, the invention pertains to improvements in methods and apparatus for mass spectrographic analysis of wafer and semiconductor die surface layers.
2. State of the Art
Secondary ion mass spectrometry (SIMS) is known as a method for determining particular constituents of a semiconductor material and providing a quantitative measurement of each.
Generally, this method involves bombarding a sample with xe2x80x9cprimaryxe2x80x9d ions, e.g. oxygen ions, measuring the intensities of secondary ions emitted or sputtered from the sample, and calculating the quantity of each conductive impurity based on the secondary emission as compared to the emission of standard materials. The sputtering and analysis is typically conducted in an ultra-high-vacuum environment.
SIMS may be used to achieve parts-per-billion (ppb) detection limits for bulk analysis and for determining material composition as a function of depth, provided the sample size is sufficiently large. The extreme sensitivity of SIMS results from its ability to xe2x80x9cconsumexe2x80x9d large amounts of sample material, and thus process a large number of atoms to detect. However, because of the high rate of material consumption from a very small sample, dynamic SIMS is generally not appropriate for analysis of a very thin oxide surface layer of a semiconductor die and plurality of semiconductor dice in wafer form. Typical semiconductor contaminants may include lithium, boron, sodium, potassium, iron, sulfur, and carbon, all of which are found in the oxide layer on the semiconductor die surface. For the case of surface contaminants on silicon, the oxide layer is generally not more than about 15 xc3x85 thick. However, several minutes are required to obtain a sufficient number of data points at the required analyte masses, so the method is not useful for this application as the oxide layer will be quickly consumed.
It is desirable to be able to detect the concentrations of boron, lithium and sodium to less than about 1xc3x97106 atoms per square centimeter of semiconductor die surface area. These detection limits are considerably lower than currently obtainable.
U.S. Pat. No. 4,874,946 of Kazmerski discloses a method and apparatus for mapping the chemical composition of a solid device, using a rasterable SIMS mass analyzer.
U.S. Pat. No. 4,611,120 of Bancroft et al. discloses a method for suppressing molecular ions in the secondary ion mass spectra of a commercial SIMS instrument.
U.S. Pat. No. 5,521,377 of Kataoka et al. discloses a method for analysis of a solid in a planar or depth-wise direction using sputtering with two ionizing beams and detecting a two-atom composite ion.
U.S. Pat. Nos. 5,502,305 of Kataoka and 5,442,174 of Kataoka et al. disclose methods for analysis of a solid in a planar or depth-wise direction using sputtering with an ionizing beam and detecting a three-atom composite ion.
U.S. Pat. No. 5,332,879 of Radhakrishnan et al. discloses the use of a pulsed laser beam to remove contaminant metals from the surface of a polyimide layer. The disclosure indicated high surface metal removal with xe2x80x9cminimalxe2x80x9d removal of the polyimide, i.e. 250-500 xc3x85 per pulse. Such ablation rates are far greater than useful in the analysis of surface contaminants in semiconductor devices, where the surface oxide layer is typically only about 15 xc3x85 in depth.
Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) has also been found useful for bulk analysis of materials, provided the sample size is sufficiently large. The TOF-SIMS instrument directly measures the speeds of secondary ions by measuring the time taken to travel a given distance. Knowing the ion""s energy, which is defined by the spectrometer""s acceleration voltage, its mass can then be calculated. Typically, the time intervals are defined as the difference in time between pulsing the ion gun and the ion arrival at the detector. The mass range is then calibrated using at least three known mass peaks.
TOF-SIMS instruments have been found to provide some of the lowest detection limits in surface analysis, typically even lower than total reflection X-ray fluorescence (TXRF) with vapor phase decomposition (VPD). For the TOF-SIMS instrument, some representative detection limits are  less than 1xc3x97108 atoms/cm2 for lithium, boron and sodium, and  less than 1xc3x97109 atoms/cm2 for iron.
The TXRF instrument, on the other hand, is incapable of detecting elements lighter than sulfur, so critical elements such as sodium, carbon, lithium and boron cannot be detected.
Thus, the TOF-SIMS method would appear to be potentially useful for surface analysis, but instrumental constraints limit the sampling area to about 100xc3x97100 xcexcm, and sampling of a relatively shallow oxide layer over the 100xc3x97100 xcexcm area does not produce sufficient sample material for achieving the desired detection limits.
For TOF-SIMS, the detection limits are determined by the transmission and exceptance of the mass spectrometer, the sputter and ionization yield of the analyte, and the amount of material consumed during the analysis. These parameters may be categorized as the useful yield of the mass spectrometer and volume of analyte. Sampling of the maximum raster area of 100xc3x97100 xcexcm to a depth of about 13 xc3x85 will produce about 3xc3x971011 particles. This is equivalent to between 0.3 to 30 (thirty) counts of a measured component at the 1 ppm level depending upon the ionization yields. It is critical to semiconductor device manufacture that bulk concentrations of some contaminants as low as 0.01 ppm and even 1 ppb be accurately detectable. Thus, current detection limits for certain contaminants must be reduced by a factor on the order of about 100 or more.

U.S. Pat. No. 5,087,815 of Schultz et al. discloses a method and apparatus for a TOF-SIMS isotopic ratio determination of elements on a surface.
The present invention provides a method for substantially increasing the sensitivity of a mass spectrographic analytical method for determining contaminant levels in the surface oxide layer of a semiconductor die or semiconductor dice in wafer form.
The present invention further provides a method for increasing the sensitivity of surface contaminant analysis of an oxide layer of a wafer by secondary ion mass spectroscopy (SIMS).
The present invention additionally provides a method for increasing the sensitivity of surface contaminant analysis by time-of-flight secondary ion mass spectroscopy (TOF-SIMS).
Related to the present invention is providing a method for determining the bulk concentration of contaminants in a surface oxide layer of a semiconductor material by SIMS or TOF-SIMS, wherein the surface area which may be sampled may be much greater than the electrostatic raster limits of the sputtering primary beam, and/or acceptance area limits of the spectrometer.
The present invention further includes apparatus for achieving the desired rastered sputtering and analysis of an enlarged area of a semiconductor wafer.
The present invention includes a method and means which enable sputtering to a uniform sampling depth and maintaining mass resolution irrespective of warpage or other non-planarity of a wafer. Thus, contaminant analysis of semiconductor wafers and semiconductor dice by SIMS or TOF-SIMS may be limited to the surface oxide layer. Additionally, advantages and novel features of this invention are set forth in part in the description infra. These advantages and features will become apparent to those skilled in the art upon examination of the following specification and drawings, or may be learned by practice of the invention. The various combinations of apparatus and/or methods which comprise the invention are pointed out in the appended claims.
In accordance with this disclosure, a first aspect of the invention comprises the steps of:
(a) scan-sputtering a large area of a semiconductor wafer surface while continuously moving, i.e. scanning the wafer with a supporting mechanical stage in a first direction, and mechanically rastering the wafer in a second direction, whereby the scanning speed is controlled to limit the sputtering to a sampling depth not generally exceeding the depth Q of the surface oxide layer, and the rate of sputtered secondary ionic emission directed to the SIMS detector simultaneously satisfies the SIMS consumption rate; the total sputtered area is generally at least about 104 xcexcm2 for a surface oxide layer sputtered to a depth of about 15 xc3x85;
(b) directing a stream of secondary ions produced by the primary ionizing beam into a SIMS for mass spectrographic analysis at a rate satisfying the SIMS sample consumption rate for a time period sufficient for high analytical sensitivity; and
(c) computing the total mass of each of selected detected ions.
In a second aspect of the invention, a combination of mechanical scanning/rastering and primary beam electrostatic rastering is used to move the sputtering and sampling operations over the wafer surface at a speed responsive to a controlled sampling depth and sample consumption over a large area.
In a third aspect of the invention, mechanical scanning and rastering is used to sample a large area of a semiconductor material with a continuous sputtering and analysis by a time-of-flight secondary ion mass spectrometer (TOF-SIMS).
In a fourth aspect of the invention, a combination of mechanical rastering and primary beam rastering, e.g. electrostatic rastering is applied to a time-of-flight secondary ion mass spectrometer (TOF-SIMS) whereby a large area of controlled limited depth may be sputtered at a speed which simultaneously (a) limits total sputtering depth generally to the surface oxide layer and (b) provides a large quantity of secondary ions for high resolution of measured atoms.
In one preferred method, an area, typically limited by the electrostatic rastering capability of the TOF-SIMS instrument to about 100 xcexcm2, is repetitively sputtered and analyzed. The primary beam is then moved to a new area by mechanical stage rastering, and the new area is repetitively sputtered and analyzed using electrostatic rastering. The process is repeated until sufficient sample material is consumed for each xe2x80x9cslicexe2x80x9d of the surface oxide layer to provide the desired detection limits. The total area sampled is limited only by the total area of the wafer or die being analyzed, and the time available for analysis.
Thus, the detection limits are reduced to low levels. For example, the detection limits of lithium, boron, and sodium may be extended to less than about 1xc3x97106 atoms/cm2.
The detection limits of trace iron may be at somewhat higher concentrations because of the peak shape of the neighboring SiO2.
In a variant of the above method, mechanical rastering is performed such that a thin slice of an enlarged area, e.g. 400-1600 xcexcm2 is sputtered and analyzed before the next slice is sputtered and analyzed.
In each of the embodiments described and illustrated of the present invention, it should be kept in mind that removal of the surface layer by sputtering is conducted in a continuous scanning motion rather than by stepping along a series of stationary raster points. The sputter depth is controlled by the scanning speed, and the sputter rate (mass per unit time) is controlled by the ion gun characteristics.
Because of the much greater area which is rastered, sputtered and analyzed, the analysis is more sensitive to non-planarity, e.g. warpage, of the wafer. Non-uniform extraction fields and some loss of mass resolution inaccuracy in analysis may result. This would have adverse effects on detection limits of Fe since the neighboring peak of Si2 is at the same nominal mass as Fe and will overlap Fe without sufficient mass resolution.
Thus, in another aspect of the present invention, a method and apparatus are provided for counteracting the effects of wafer warpage (non-planarity) upon analytical results. In accordance with the invention, a non-invasive laser interferometer is incorporated into the SIMS or TOF-SIMS spectrometer to measure non-planarity as a function of wafer location, and permit correction of the stage elevation or the sample electrical potential relative to the extraction potential. The sample elevation measurements may be made prior to the mass spectrometric analysis and then used to control e.g. the stage elevation during rastering/sputtering, or the sample elevation measurements may be made during the rastering/sputtering operation for continuous in-situ correction. Because of the required physical separation, e.g. about 1-3 cm., of the sputter beam and the interferometer laser beam in the latter case, the sample surface elevation at the sputter location may be calculated and correction made therefor by a computer program of the process controller, based on distant measurements and assuming a particular warpage shape. Alternatively, the wafer may be first scanned by the interferometer; the data may be stored and used to provide correction during the sputtering/analyzing operation.